Membrane probing system

ABSTRACT

A substrate, preferably constructed of a ductile material and a tool having the desired shape of the resulting device for contacting contact pads on a test device is brought into contact with the substrate. The tool is preferably constructed of a material that is harder than the substrate so that a depression can be readily made therein. A dielectric (insulative) layer, that is preferably patterned, is supported by the substrate. A conductive material is located within the depressions and then preferably lapped to remove excess from the top surface of the dielectric layer and to provide a flat overall surface. A trace is patterned on the dielectric layer and the conductive material. A polyimide layer is then preferably patterned over the entire surface. The substrate is then removed by any suitable process.

This application is a continuation of U.S. Pat. application Ser. No.09/814,584, filed on Mar. 22, 2001, now U.S. Pat. No. 6,860,009 which isa continuation of U.S. patent application Ser. No. 09/115,571 filed onJul. 14, 1998, now U.S. Pat. No. 6,256,882.

BACKGROUND OF THE INVENTION

The present invention relates to probe assemblies of the type commonlyused for testing integrated circuits (IC) and, in particular, thepresent invention relates to a membrane probing assembly having contactswhich scrub, in a locally controlled manner, across the respectiveinput/output conductors of each device so as to reliably wipe clear thesurface oxides that are normally found on those conductors therebyensuring good electrical connection between the probing assembly andeach device.

The trend in electronic production has been toward increasingly smallergeometries particularly in integrated circuit technology wherein a verylarge number of discrete circuit elements are fabricated on a singlesubstrate or “wafer.” After fabrication, this wafer is divided into anumber of rectangular-shaped chips or “dice” where each die presents arectangular or other regular arrangement of metallized contact padsthrough which input/output connections are made. Although each die iseventually packaged separately, for efficiency sake, testing of thecircuit formed on each die is preferably performed while the dies arestill joined together on the wafer. One typical procedure is to supportthe wafer on a flat stage or “chuck” and to move the wafer in X, Y and Zdirections relative to the head of the probing assembly so that thecontacts on the probing assembly move from die to die for consecutiveengagement with each die. Respective signal, power and ground lines arerun to the probing assembly from the test instrumentation thus enablingeach circuit to be sequentially connected to the test instrumentation.

One conventional type of probing assembly used for testing integratedcircuits provides contacts that are configured as needle-like tips.These tips are mounted about a central opening formed in a probe card soas to radially converge inwardly and downwardly through the opening.When the wafer is raised beyond that point where the pads on the waferfirst come into contact with these tips, the tips flex upwardly so as toskate forwardly across their respective pads thereby removing oxidebuildup on the pads.

The problem with this type of probing assembly is that the needle-liketips, due to their narrow geometry, exhibit high inductance so thatsignal distortion is large in high frequency measurements made throughthese tips. Also, these tips can act in the manner of a planing tool asthey wipe across their respective pads, thereby leading to excessive paddamage. This problem is magnified to the extent that the probe tips bendout of shape during use or otherwise fail to terminate in a common planewhich causes the more forward ones of the tips to bear down too heavilyon their respective pads. Also, it is impractical to mount these tips atless than 100 micron center-to-center spacing or in a multi-rowgrid-like pattern so as to accommodate the pad arrangement of moremodern, higher density dies. Also, this type of probing assembly has ascrub length of the needle tips of 25 microns or more, which increasesthe difficulty of staying within the allowed probing area.

In order to reduce inductive losses, decrease pad wear and accommodatesmaller device geometries, a second type of probing assembly has beendeveloped that uses a flexible membrane structure for supporting theprobing contacts. In this assembly, lead lines of well-defined geometryare formed on one or more plies of flexible insulative film, such aspolyimide or MYLAR™. If separate plies are used, these plies are bondedtogether to form, for example, a multilayered transmission linestructure. In the central portion of this flexible structure ormembrane, each conductive line is terminated by a respective probingcontact which is formed on, and projects outwardly from, an outer faceof the membrane. These probing contacts are arranged in a predeterminedpattern that matches the pattern of the device pads and typically areformed as upraised bumps for probing the flat surfaces conventionallydefined by the pads. The inner face of the membrane is supported on asupporting structure. This structure can take the form, for example, ofa truncated pyramid, in which case the inner face of the center portionof the membrane is supported on the truncated end of the support whilethe marginal portions of the membrane are drawn away from the centerportion at an angle thereto so as to clear any upright components thatmay surround the pads on the device.

With respect to the membrane probing assembly just described, excessiveline inductance is eliminated by carefully selecting the geometry of thelead lines, and a photolithographic process is preferably used to enablesome control over the size, spacing, and arrangement, of the probingcontacts so as to accommodate higher density configurations. However,although several different forms of this probing assembly have beenproposed, difficulties have been encountered in connection with thistype of assembly in reducing pad wear and in achieving reliable clearingof the oxide layer from each of the device pads so as to ensure adequateelectrical connection between the assembly and the device-under-test.

One conventional form of membrane probing assembly, for example, isexemplified by the device shown in Rath European Patent Pub. No.259,163A2. This device has the central portion of the sheet-likemembrane mounted directly against a rigid support. This rigid support,in turn, is connected by a resilient member comprising an elastomeric orrubber block to the main body of the assembly so that the membrane cantilt to match the tilt of the device. Huff U.S. Pat. No. 4,918,383 showsa closely related device wherein radially extending leaf springs permitvertical axis movement of the rigid support while preventing it fromtilting so that there is no slippage or “misalignment” of the contactbumps on the pads and further so that the entire membrane will shiftslightly in the horizontal plane to allow the contacts to “scrub” acrosstheir respective pads in order to clear surface oxides from these pads.

In respect to both of these devices, however, because of manufacturingtolerances, certain of the contact bumps are likely to be in a recessedposition relative to their neighbors and these recessed bumps will nothave a satisfactory opportunity to engage their pads since they will bedrawn away from their pads by the action of their neighbors on the rigidsupport. Furthermore, even when “scrub” movement is provided in themanner of Huff, the contacts will tend to frictionally cling to thedevice as they perform the scrubbing movement, that is, there will be atendency for the pads of the device to move in unison with the contactsso as to negate the effect of the contact movement. Whether anyscrubbing action actually occurs depends on how far the pads can move,which depends, in turn, on the degree of lateral play that exists as aresult of normal tolerance between the respective bearing surfaces ofthe probe head and chuck. Hence this form of membrane probing assemblydoes not ensure reliable electrical connection between each contact andpad.

A second conventional form of membrane probing assembly is exemplifiedby the device shown in Barsotti European Patent Pub. No. 304,868A2. Thisdevice provides a flexible backing for the central or contact-carryingportion of the flexible membrane. In Barsotti, the membrane is directlybacked by an elastomeric member and this member, in turn, is backed by arigid support so that minor height variations between the contacts orpads can be accommodated. It is also possible to use positive-pressureair, negative-pressure air, liquid or an unbacked elastomer to provideflexible backing for the membrane, as shown in Gangroth U.S. Pat. No.4,649,339, Ardezzone U.S. Pat. No. 4,636,772, Reed, Jr. et al. U.S. Pat.No. 3,596,228 and Okubo et al. U.S. Pat. No. 5,134,365, respectively.These alternative devices, however, do not afford sufficient pressurebetween the probing contacts and the device pads to reliably penetratethe oxides that form on the pad surfaces.

In this second form of membrane probing assembly, as indicated in Okubo,the contacts may be limited to movement along the Z-axis in order toprevent slippage and resulting misalignment between the contacts andpads during engagement. Thus, in Barsotti, the rigid support underlyingthe elastomeric member is fixed in position although it is also possibleto mount the support for Z-axis movement in the manner shown in HuffU.S. Pat. No. 4,980,637. Pad damage is likely to occur with this type ofdesign, however, because a certain amount of tilt is typically presentbetween the contacts and the device, and those contacts angled closestto the device will ordinarily develop much higher contact pressures thanthose which are angled away. The same problem arises with the relatedassembly shown in European Patent Pub. No. 230,348A2 to Garretson, eventhough in the Garretson device the characteristic of the elastomericmember is such as to urge the contacts into lateral movement when thosecontacts are placed into pressing engagement with their pads. Yetanother related assembly is shown in Evans U.S. Pat. No. 4,975,638 whichuses a pivotably mounted support for backing the elastomeric member soas to accommodate tilt between the contacts and the device. However, theEvans device is subject to the friction clinging problem alreadydescribed insofar as the pads of the device are likely to cling to thecontacts as the support pivots and causes the contacts to shiftlaterally.

Yet other forms of conventional membrane probing assemblies are shown inCrumly U.S. Pat. No. 5,395,253, Barsotti et al. U.S. Pat. No. 5,059,898and Evans et al. U.S. Pat. No. 4,975,638. In Crumly, the center portionof a stretchable membrane is resiliently biased to a fully stretchedcondition using a spring. When the contacts engage their respectivepads, the stretched center portion retracts against the spring to apartially relaxed condition so as to draw the contacts in radial scrubdirections toward the center of the membrane. In Barsotti, each row ofcontacts is supported by the end of a respective L-shaped arm so thatwhen the contacts in a row engage their respective pads, thecorresponding arm flexes upwardly and causes the row of contacts tolaterally scrub simultaneously across their respective pads. In bothCrumly and Barsotti, however, if any tilt is present between thecontacts and the device at the time of engagement, this tilt will causethe contacts angled closest to the device to scrub further than thoseangled further away. Moreover, the shorter contacts will be forced tomove in their scrub directions before they have had the opportunity toengage their respective pads due to the controlling scrub action oftheir neighboring contacts. A further disadvantage of the Crumly device,in particular, is that the contacts nearer to the center of the membranewill scrub less than those nearer to the periphery so that scrubeffectiveness will vary with contact position.

In Evans et al. U.S. Pat. No. 5,355,079 each contact constitutes aspring metal finger, and each finger is mounted so as to extend in acantilevered manner away from the underlying membrane at a predeterminedangle relative to the membrane. A similar configuration is shown inHiggins U.S. Pat. No. 5,521,518. It is difficult, however, to originallyposition these fingers so that they all terminate in a common plane,particularly if a high density pattern is required. Moreover, thesefingers are easily bent out of position during use and cannot easily berebent back to their original position. Hence, certain ones of thefingers are likely to touch down before other ones of the fingers, andscrub pressures and distances are likely to be different for differentfingers. Nor, in Evans at least, is there an adequate mechanism fortolerating a minor degree of tilt between the fingers and pads. AlthoughEvans suggests roughening the surface of each finger to improve thequality of electrical connection, this roughening can cause undueabrasion and damage to the pad surfaces. Yet a further disadvantage ofthe contact fingers shown in both Evans and Higgins is that such fingersare subject to fatigue and failure after a relatively low number of“touchdowns” or duty cycles due to repeated bending and stressing.

Referring to FIG. 1, Cascade Microtech, Inc. of Beaverton, Oreg. hasdeveloped a probe head 40 for mounting a membrane probing assembly 42.In order to measure the electrical performance of a particular die area44 included on the silicon wafer 46, the high-speed digital lines 48and/or shielded transmission lines 50 of the probe head are connected tothe input/output ports of the test instrumentation by a suitable cableassembly, and the chuck 51 which supports the wafer is moved in mutuallyperpendicular X, Y, Z directions in order to bring the pads of the diearea into pressing engagement with the contacts included on the lowercontacting portion of the membrane probing assembly.

The probe head 40 includes a probe card 52 on which the data/signallines 48 and 50 are arranged. Referring to FIGS. 2-3, the membraneprobing assembly 42 includes a support element 54 formed ofincompressible material such as a hard polymer. This element isdetachably connected to the upper side of the probe card by four Allenscrews 56 and corresponding nuts 58 (each screw passes through arespective attachment arm 60 of the support element, and a separatebacking element 62 evenly distributes the clamping pressure of thescrews over the entire back side of the supporting element). Inaccordance with this detachable connection, different probing assemblieshaving different contact arrangements can be quickly substituted foreach other as needed for probing different devices.

Referring to FIGS. 3-4, the support element 54 includes a rearward baseportion 64 to which the attachment arms 60 are integrally joined. Alsoincluded on the support element 54 is a forward support or plunger 66that projects outwardly from the flat base portion. This forward supporthas angled sides 68 that converge toward a flat support surface 70 so asto give the forward support the shape of a truncated pyramid. Referringalso to FIG. 2, a flexible membrane assembly 72 is attached to thesupport after being aligned by means of alignment pins 74 included onthe base portion. This flexible membrane assembly is formed by one ormore plies of insulative sheeting such as KAPTON™ sold by E.I. Du Pontde Nemours or other polyimide film, and flexible conductive layers orstrips are provided between or on these plies to form the data/signallines 76.

When the support element 54 is mounted on the upper side of the probecard 52 as shown in FIG. 3, the forward support 66 protrudes through acentral opening 78 in the probe card so as to present the contacts whichare arranged on a central region 80 of the flexible membrane assembly insuitable position for pressing engagement with the pads of the deviceunder test. Referring to FIG. 2, the membrane assembly includes radiallyextending arm segments 82 that are separated by inwardly curving edges84 that give the assembly the shape of a formee cross, and thesesegments extend in an inclined manner along the angled sides 68 therebyclearing any upright components surrounding the pads. A series ofcontact pads 86 terminate the data/signal lines 76 so that when thesupport element is mounted, these pads electrically engage correspondingtermination pads provided on the upper side of the probe card so thatthe data/signal lines 48 on the probe card are electrically connected tothe contacts on the central region.

A feature of the probing assembly 42 is its capability for probing asomewhat dense arrangement of contact pads over a large number ofcontact cycles in a manner that provides generally reliable electricalconnection between the contacts and pads in each cycle despite oxidebuildup on the pads. This capability is a function of the constructionof the support element 54, the flexible membrane assembly 72 and theirmanner of interconnection. In particular, the membrane assembly is soconstructed and connected to the support element that the contacts onthe membrane assembly preferably wipe or scrub, in a locally controlledmanner, laterally across the pads when brought into pressing engagementwith these pads. The preferred mechanism for producing this scrubbingaction is described in connection with the construction andinterconnection of a preferred membrane assembly 72 a as best depictedin FIGS. 6 and 7 a-7 b.

FIG. 6 shows an enlarged view of the central region 80 a of the membraneassembly 72 a. In this embodiment, the contacts 88 are arranged in asquare-like pattern suitable for engagement with a square-likearrangement of pads. Referring also to FIG. 7 a, which represents asectional view taken along lines 7 a-7 a in FIG. 6, each contactcomprises a relatively thick rigid beam 90 at one end of which is formeda rigid contract bump 92. The contact bump includes thereon a contactingportion 93 which comprises a nub of rhodium fused to the contact bump.Using electroplating, each beam is formed in an overlapping connectionwith the end of a flexible conductive trace 76 a to form a jointtherewith. This conductive trace in conjunction with a back-planeconductive layer 94 effectively provides a controlled impedancedata/signal line to the contact because its dimensions are establishedusing a photolithographic process. The backplane layer preferablyincludes openings therein to assist, for example, with gas ventingduring fabrication.

The membrane assembly is interconnected to the flat support surface 70by an interposed elastomeric layer 98, which layer is coextensive withthe support surface and can be formed by a silicone rubber compound suchas ELMER'S STICK-ALL™ made by the Borden Company or Sylgard 182 by DowCorning Corporation. This compound can be conveniently applied in apaste-like phase which hardens as it sets. The flat support surface, aspreviously mentioned, is made of incompressible material and ispreferably a hard dielectric such as polysulfone or glass.

In accordance with the above-described construction, when one of thecontacts 88 is brought into pressing engagement with a respective pad100, as indicated in FIG. 7 b, the resulting off-center force on therigid beam 90 and bump 92 structure causes the beam to pivot or tiltagainst the elastic recovery force provided by the elastomeric pad 98.This tilting motion is localized in the sense that a forward portion 102of the beam moves a greater distance toward the flat support surface 70than a rearward portion 104 of the same beam. The effect is such as todrive the contact into lateral scrubbing movement across the pad as isindicated in FIG. 7 b with a dashed-line and solid-line representationshowing the beginning and ending positions, respectively, of the contacton the pad. In this fashion, the insulative oxide buildup on each pad isremoved so as to ensure adequate contact-to-pad electrical connections.

FIG. 8 shows, in dashed line view, the relative positions of the contact88 and pad 100 at the moment of initial engagement or touchdown and, insolid-line view, these same elements after “overtravel” of the pad by adistance 106 in a vertical direction directly toward the flat supportsurface 70. As indicated, the distance 108 of lateral scrubbing movementis directly dependent on the vertical deflection of the contact 88 or,equivalently, on the overtravel distance 106 moved by the pad 100.Hence, since the overtravel distance for each contact on the centralregion 80 a will be substantially the same (with differences arisingfrom variations in contact height), the distance of lateral scrubbingmovement by each contact on the central region will be substantiallyuniform and will not, in particular, be affected by the relativeposition of each contact on the central region.

Because the elastomeric layer 98 is backed by the incompressible supportsurface 70, the elastomeric layer exerts a recovery force on eachtilting beam 90 and thus each contact 93 to maintain contact-to-padpressure during scrubbing. At the same time, the elastomeric layeraccommodates some height variations between the respective contacts.Thus, referring to FIG. 9 a, when a relatively shorter contact 88 a issituated between an immediately adjacent pair of relatively tallercontacts 88 b and these taller contacts are brought into engagement withtheir respective pads, then, as indicated in FIG. 9 b, deformation bythe elastomeric layer allows the smaller contact to be brought intoengagement with its pad after some further overtravel by the pads. Itwill be noted, in this example, that the tilting action of each contactis locally controlled, and the larger contacts are able, in particular,to tilt independently of the smaller contact so that the smaller contactis not urged into lateral movement until it has actually touched down onits pad.

Referring to FIGS. 10 and 11, the electroplating process to constructsuch a beam structure, as schematically shown in FIG. 8, includes theincompressible material 68 defining the support surface 70 and thesubstrate material attached thereon, such as the elastomeric layer 98.Using a flex circuit construction technique, the flexible conductivetrace 76 a is then patterned on a sacrificial substrate. Next, apolyimide layer 77 is patterned to cover the entire surface of thesacrificial substrate and of the traces 76 a, except for the desiredlocation of the beams 90 on a portion of the traces 76 a. The beams 90are then electroplated within the openings in the polyimide layer 77.Thereafter, a layer of photoresist 79 is patterned on both the surfaceof the polyimide 77 and beams 90 to leave openings for the desiredlocation of the contact bumps 92. The contact bumps 92 are thenelectroplated within the openings in the photoresist layer 79. Thephotoresist layer 79 is removed and a thicker photoresist layer 81 ispatterned to cover the exposed surfaces, except for the desiredlocations for the contacting portions 93. The contacting portions 93 arethen electroplated within the openings in the photoresist layer 81. Thephotoresist layer 81 is then removed. The sacrificial substrate layer isremoved and the remaining layers are attached to the elastomeric layer98. The resulting beams 90, contact bumps 92, and contacting portions93, as more accurately illustrated in FIG. 12, provides the independenttilting and scrubbing functions of the device.

Unfortunately, the aforementioned construction technique results in astructure with many undesirable characteristics.

First, several beams 90, contact bumps 92, and contacting portions 93(each of which may be referred to as a device) proximate one anotherresults in different localized current densities within theelectroplating bath, which in turn results in differences in the heightsof many of the beams 90, contact bumps 92, and contacting portions 93.Also, different densities of the ions within the electroplating bath and“random” variations in the electroplating bath also results indifferences in heights of many of the beams 90, contact bumps 92, andcontacting portions 93. The different heights of many of the beams 90,contact bumps 92, and contacting portions 93 is compounded three fold inthe overall height of many of the devices. Accordingly, many deviceswill have a significantly different height than other devices. Usingmembrane probes having variable device height requires more pressure toensure that all the contacting portions 93 make adequate contact withthe test device than would be required if all the devices had equaloverall height. For high density membrane probes, such as 2000 or moredevices in a small area, the cumulate effect of the additional pressurerequired for each device may exceed the total force permitted for theprobe head and probe station. The excess pressure may also result inbending and breaking of the probe station, the probe head, and/or themembrane probing assembly. In addition, the devices with the greatestheight may damage the pads on the test device because of the increasedpressure required to make suitable contact for the devices with thelowest height.

Second, the ability to decrease the pitch (spacing) between the devicesis limited by the “mushrooming” effect of the electroplating processover the edges of the polyimide 77 and photoresist layers 79 and 81. The“mushrooming” effect is difficult to control and results in a variablewidth of the beams 90, contact bumps 92, and contacting portions 93. Ifthe height of the beams 90, the contact bumps 92, or the contactingportions 93 are increased then the “mushrooming” effect generallyincreases, thus increasing the width of the respective portion. Theincreased width of one part generally results in a wider overall devicewhich in turn increases the minimum spacing between contacting portions93. Alternatively, decreasing the height of the beams 90, the contactbumps 92, or the contacting portions 93 generally decreases the width ofthe “mushrooming” effect which in turn decreases the minimum spacingbetween contacting portions 93. However, if the height of the contactingportions 93 relative to the respective beam 90 is sufficiently reduced,then during use the rearward end of the beam 90 may sufficiently tiltand contact the test device in an acceptable location, i.e., off thecontact pad.

Third, it is difficult to plate a second metal layer directly on top ofa first metal layer, such as contacting portions 93 on the contact bumps92, especially when using nickel. To provide a bond between the contactbumps 92 and the contacting portions 93, an interface seed layer such ascopper or gold is used to make an improved interconnection.Unfortunately, the interface seed layer reduces the lateral strength ofthe device due to the lower sheer strength of the interface layer.

Fourth, applying a photoresist layer over a non-uniform surface tends tobe semi-conformal in nature resulting in a non-uniform thicknesses ofthe photoresist material itself. Referring to FIG. 13, the photoresistlayer 79 (and 81) over the raised portions of the beams 90 tends to bethicker than the photoresist layer 79 (and 81) over the lower portionsof the polyimide 77. In addition, the thickness of the photoresist 79(and 81) tends to vary depending on the density of the beams 90.Accordingly, regions of the membrane probe that have a denser spacing ofdevices, the photoresist layer 79 (and 81) will be thicker on averagethan regions of the membrane probe that have a less dense spacing ofdevices. During the exposing and etching processing of the photoresistlayer 79 (and 81), the duration of the process depends on the thicknessof the photoresist 79 (or 81). With variable photoresist thickness it isdifficult to properly process the photoresist to provide uniformopenings. Moreover, the thinner regions of photoresist layer 79 (or 81)will tend to be overexposed resulting in variably sized openings. Also,the greater the photoresist layer thickness 79 (or 81) the greater thevariability in its thickness. Accordingly, the use of photoresistpresents many processing problems.

Fifth, separate alignment processes are necessary to align the beams 90on the traces 76 a, the contact bumps 92 on the beams 90, and thecontacting portions 93 on the contact bumps 92. Each alignment processhas inherent variations that must be accounted for in sizing each part.The minimum size of the contacting portions 93 is defined primarily bythe lateral strength requirements and the maximum allowable currentdensity therein. The minimum size of the contacting portions 93,accounting for the tolerances in alignment, in turn defines the minimumsize of the contact bumps 92 so that the contacting portions 93 aredefinitely constructed on the contact bumps 92. The minimum size of thecontact bumps 92, in view of the contacting portions 93 and accountingfor the tolerances in alignment, defines the minimum size of the beams90 so that the contact bumps 92 are definitely constructed on the beams90. Accordingly, the summation of the tolerances of the contact bumps 92and the contacting portions 93, together with a minimum size of thecontacting portions 93, defines the minimum device size, and thusdefines the minimum pitch between contact pads.

What is desired, therefore, is a membrane probe construction techniqueand structure that results in a more uniform device height, decreasedspacing between devices, maximized lateral strength, desired geometries,and proper alignment.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks of theprior art by providing a substrate, preferably constructed of a ductilematerial A tool having the desired shape of the resulting device forcontacting contact pads on a test device is brought into contact withthe substrate. The tool is preferably constructed of a material that isharder than the substrate so that a depression can be readily madetherein. A dielectric (insulative) layer, that is preferably patterned,is supported by the substrate. A conductive material is located withinthe depressions and then preferably planarized to remove excess from thetop surface of the dielectric layer and to provide a flat overallsurface. A trace is patterned on the dielectric layer and the conductivematerial. A polyimide layer is then preferably patterned over the entiresurface. The substrate is then removed by any suitable process.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a membrane probing assembly bolted to aprobe head and a wafer supported on a chuck in suitable position forprobing by this assembly.

FIG. 2 is a bottom elevational view showing various parts of the probingassembly of FIG. 1, including a support element and flexible membraneassembly, and a fragmentary view of a probe card having data/signallines connected with corresponding lines on the membrane assembly.

FIG. 3 is a side elevational view of the membrane probing assembly ofFIG. 1 where a portion of the membrane assembly has been cut away toexpose hidden portions of the support element.

FIG. 4 is a top elevational view of an exemplary support element.

FIGS. 5 a-5 b are schematic side elevational views illustrating how thesupport element and membrane assembly are capable of tilting to matchthe orientation of the device under test.

FIG. 6 is an enlarged top elevational view of the central region of theconstruction of the membrane assembly of FIG. 2.

FIGS. 7 a-7 b are sectional views taken along lines 7 a-7 a in FIG. 6first showing a contact before touchdown and then showing the samecontact after touchdown and scrub movement across its respective pad.

FIG. 8 is a schematic side view showing, in dashed-line representation,the contact of FIGS. 7 a-7 b at the moment of initial touchdown and, insolid-line representation, the same contact after further verticalovertravel by the pad.

FIGS. 9 a and 9 b illustrate the deformation of the elastomeric layer tobring the contacts into contact with its pad.

FIG. 10 is a longitudinal sectional view of the device of FIG. 8.

FIG. 11 is a cross sectional view of the device of FIG. 8.

FIG. 12 is a more accurate pictorial view of the device shown in FIGS.10 and 11.

FIG. 13 is a detailed view of the device shown in FIG. 11 illustratingthe uneven layers that result during processing.

FIG. 14 is a pictorial view of a substrate.

FIG. 15 is a pictorial view of an exemplary embodiment of a tool, and inparticular a dimpling tool, of the present invention.

FIG. 16 is a pictorial view illustrating the tool of FIG. 15 coming intocontact with the substrate of FIG. 14.

FIG. 17 is a pictorial view of the substrate of FIG. 14 after the toolof FIG. 15 has come into contact therewith.

FIG. 18 is a sectional view of the substrate of FIG. 14 with a polyimidelayer supported thereon.

FIG. 19 is a pictorial view of the tool of FIG. 16 together with az-axis stop.

FIG. 20 is a sectional view of the substrate of FIG. 14 with a trace,conductive material in the depression, and additional polyimide layerthereon.

FIG. 21 is a pictorial view of the device of FIG. 20, inverted, with thesubstrate removed.

FIG. 22 is a breakaway sectional view of the contacting portion of FIG.21.

FIG. 23 is a schematic view illustrating one arrangement of the devicesof the present invention.

FIG. 24 is a schematic view illustrating the contact of a traditionalcontacting portion and the oxide layer of a solder bump.

FIG. 25 is a plan view of an alternative device with an elongate probingportion.

FIG. 26 is a side view of the device of FIG. 25 with an elongate probingportion.

FIG. 27 is a pictorial view of a solder bump with a mark therein as aresult of the device of FIGS. 25 and 26.

FIG. 28 is a pictorial view of another alternative probing device.

FIG. 29 is a pictorial view of a further alternative probing devicesuitable for solder bumps.

FIG. 30 is a side view of a true Kelvin connection using the devices ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The currently employed construction techniques for membrane probesinvolves starting with the flat rigid substrate to support additionallayers fabricated thereon. To decrease the pitch and provide deviceswith increased uniformity requires increasingly more complex andexpensive processing techniques. In direct contrast to the currenttechniques of constructing layers from the “bottom up” upon a supportingsubstrate, the present inventors came to the realization that by using asuitable tool a substrate may be coined to create the desired beams,contact bumps, and contacting portions. The remaining layers are thenconstructed “top down” on the beam. The substrate itself is thereafterremoved.

Referring to FIG. 14, a substrate 200 is preferably constructed from aductile material such as aluminum, copper, lead, indium, brass, gold,silver, platinum, or tantalum, with a thickness preferably between 10mills and ⅛ inch. The top surface 202 of the substrate 200 is preferablyplanar and polished for optical clarity to improve viewing, as describedlater.

Referring to FIG. 15, a tool and in particular a “dimpling” tool 210 isconstructed with a head 212 having the desired shape of the resultingdevice for contacting the contact pads on the test device. The dimplingtool 210 includes a projection 214 to connect to a dimpling machine (notshown). The tool 210 is supported by the dimpling machine with the head212 oriented to come into contact with the top surface 202 of thesubstrate 200. The tool 210 is preferably constructed of a material thatis harder than the substrate 200 so that a dimple can be readily madetherein. Suitable material for the tool 210 is, for example, tool steel,carbide, chromium, and diamond. The preferred dimpling machine is aprobe station which has accurate x, y, and z control. It is to beunderstood that any other suitable dimpling machine may likewise beused. Referring to FIG. 16, the tool 210 is pressed into contact withthe top surface 202 of the substrate 200 resulting in a depression 216matching the shape of the tool 210 upon its removal from the substrate200, as shown in FIG. 17. The tool 210 is used to create a plurality ofdepressions 216 in the substrate 200 matching the desired pattern, suchas the pattern shown in FIG. 6. Conversely, the tool 210 can be heldstationary and the substrate 200 can be moved in the z-direction untilthe top surface 202 of the substrate is pressed into contact with thetool 210 resulting in the same depression 216 matching the shape of thetool 210 upon its removal from the substrate 200, as shown in FIG. 17.

Referring to FIG. 18, a polyimide layer 220 is patterned around thedepressions 216. It is to be understood that any other suitableinsulative layer or dielectric layer may likewise be used. In theprocess of patterning the polyimide layer 220, it is somewhat difficultto remove the polyimide from the depressions 216 during the exposing andetching process for the polyimide layer 220. This is especially truewhen the depressions 216 are relatively deep with steeply inclinedsides. Alternatively, the polyimide layer 220 may be patterned on thetop surface 202 of the substrate 200 with openings located therein wherethe depressions 216 are desired. Thereafter, the tool 210 is used tocreate the depressions 216 in the substrate 200 through the openingsprovided in the polyimide layer 220. This alternative techniqueeliminates the difficult process of adequately removing the polyimidelayer 220 from the depressions 216.

It is expensive to manufacture masks for exposing the polyimide layer220 that have tolerances sufficient to precisely align the openings forthe depressions 216. The tool 210, in combination with the dimplingmachine, can be aligned to the actual location of one of the openingsthat results from exposing and etching the polyimide layer 220 with arelatively inexpensive, and somewhat inaccurate mask. The presentinventors came to the realization that localized regions of the mask,and thus the openings resulting therefrom, tend to be relatively wellaligned for purposes of dimpling. Likewise, regions of the mask distantfrom one another tend not to be relatively well aligned for purposes ofdimpling. Accordingly, automatically dimpling the substrate 200 to matchan anticipated pattern with many depressions 216 distant from oneanother, with an accurate dimpling machine, will result in the dimplingtool not accurately being aligned with the openings at regions distantfrom the initial alignment point. To improve the accuracy of thealignment process the present inventors came to the realization that thedimpling machine may be realigned to the actual openings in thepolyimide layer 220 at different remote locations, so that eachlocalized region is relatively accurately aligned, while the overallalignment may be somewhat off. In this manner a relatively inexpensivemask may be used.

Preferably the dimpling machine includes accurate z-axis movement sothat the depth of each depression is identical, or substantiallyidentical.

Referring to FIG. 19, if sufficiently accurate z-axis movement is notavailable then an alternative dimpling tool 240 with a built in z-axisstop 242 may be used. The z-axis stop 242 is a projection extendingoutward from the head 244 that comes to rest on the top surface of thepolyimide 220 or top surface 202 of the substrate 200. The z-axis stop242 is positioned with respect to the head 244 such that the properdepth is obtained, taking into account whether or not the polyimidelayer 220 is previously patterned before using the dimpling tool 240.

Referring to FIG. 20, a conductive material 250 is electroplated ontothe polyimide 220 and substrate 200 thereby filling up the depressions216 with the conductive material 250, such as nickel and rhodium. It isto be understood that any other suitable technique may be used to locateconductive material within the depressions 216. The conductive material250 is then preferably lapped to remove excess from the top surface ofthe polyimide layer 220 and to provide a flat overall surface. Thepreferred lapping process is a chemical-mechanical planarizationprocess. A trace 252 is patterned on the polyimide layer 220 and theconductive material 250. The trace 252 is preferably a good conductorsuch as copper, aluminum, or gold. A polyimide layer 254 is thenpatterned over the entire surface. Further layers of metal anddielectric may be formed. The substrate 200 is then removed by anysuitable process, such as etching with hydrochloric acid (HCL 15%) orsulfuric acid (H₂SO₄). Hydrochloric acid and sulfuric acid are notreactive with the polyimide layer 220 nor the conductive material 250,such as nickel or rhodium. It is to be understood that the polyimidelayer 254 may alternatively be any suitable insulator or dielectriclayer.

Referring to FIG. 21, the contacting portion 260 of the resulting deviceis preferably selected to have a low contact resistance so that a goodelectrical connection may be made with the test device. While nickel hasa relatively low contact resistance, rhodium has an even lower contactresistance and is more resistant to wear than nickel. Accordingly, thedepressions 216 are preferably coated with a layer of rhodium. Usingnormal processing techniques the thickness of rhodium is limited toapproximately 5 microns. The resulting device includes an exterior layerof rhodium, and in particular the contacting portion 260, which is thenfilled with the remaining conductive material, such as nickel or anonconductive fill. The conductive material need not fill the entiredepression.

The aforementioned “top-down” construction process provides numerousadvantages over the traditional “bottom-up” processing technique ofconstructing layers upon a supporting substrate. These advantages alsopermit the capability of constructing devices with improvedcharacteristics.

First, there are no limitations to the height of the resulting deviceswhich were previously imposed by limitations of photoresist processing.The ability to construct devices having any suitable height alsorelieves the limitations imposed by attempting to electroplate into atall narrow openings in photoresist, which is difficult.

Second, the elevation of the contacting portions 260 of the devices isextremely uniform because it is defined solely by the tooling process,which is mechanical in nature. Different localized current densities ofthe electroplating bath, different densities of the ions within theelectroplating bath, and “random” variations in the electroplating bathare eliminated from impacting the overall shape and height of theresulting devices. With substantially uniform elevation of the devices,less force is required for the devices to make adequate contact with thetest device which, in turn, decreases the likelihood of bending andbreaking the probe station, the probe head, and/or the membrane probingassembly. Also, the substantially uniform elevation of the devicesdecreases the likelihood of damaging contact pads on the test devicewith excessive pressure.

Third, the contacting portion 260 of the devices are stronger becausethe device is constructed of a single homogenous material during onedepositing process requiring no interfacial layers, as previouslyrequired for the multiple processing steps. This permits reducing thesize of the contacting portions to the limitation of the maximum currentdensity allowable therein during testing and not the minimum sheer forceof the interfacial layers.

Fourth, the shape of the resulting devices are customizable toeffectively probe different materials. The shape of the device may havesteep sidewall angles, such as 85 degrees, while still providingmechanical strength, stability, and integrity. The steep sidewallspermit narrower devices to be constructed which allows for a greaterdensity of devices for increasingly denser arrangements of contact padson the test device. Moreover, the angle of the sidewalls are notdependent (e.g. independent) on the crystalline structure of thesubstrate.

Fifth, the shape of the contacting portion is known precisely, and isuniform between devices, which permits uniform contact with the contactpads of the test device.

Sixth, the alignment of the different portions of the resulting deviceare exactly uniform between devices because each device was constructedusing the same tooling process. With exact alignment of the lowerportions of each device (beam and contact bump) in relation to thecontacting portion, there is no need to provide additional leeway toaccommodate processing variations inherent in photoresist processes andin electroplating processes. Also, the “mushrooming” effect of theelectroplating process is eliminated which also reduces the requiredsize of the device. The alignment variability reduction, and virtualelimination, of different devices 300 allows a significantly decreasedpitch to be obtained, suitable for contact pads on the test device thathave increased density.

Seventh, the shape of the resulting devices may be tailor shaped toprovide optimal mechanical performance. To provide the scrubbingfunction, as described in the background portion, the device should havea beam and bump structure that tilts upon contact. The device 300 mayinclude an inclined surface 304 between its tail 302 and the contactingportion 260. The inclined surface 304 provides for increased strengthalong portions of the length of the device 300 which permits the tail302 to be thinner than its head 306. The torque forces applied to thedevice 300 during the tilting process of the device 300 tend to decreaseover the length of the device 300 which has a correspondingly thinnermaterial defined by the inclined surface 304. With a thinner tail 302and material proximate the tail 302, the tail 302 of the device 300 hasless likelihood of impacting the test device if excess tiling occurs.The improved shape of the device 300 also decreases the amount of metalmaterial required.

Eighth, “look-up” cameras are used to obtain an image of the lowerportion of the membrane probe to determine the precise location of thedevices 300 relative to the contact pads on the test device. Using“look-up” cameras permits automatic alignment of the membrane devicesrelative to the contact pads so that automatic testing may be performed.In order to obtain an image of the devices 300 on the membrane probe the“look-up” cameras normally utilize light to illuminate the devices 300.Unfortunately, the traditional planar processing techniques result inrelatively flat surfaces on the beams, contact bumps, and contactingportions, in a perpendicular orientation to the “look up” cameras eachof which reflects light back to the “look-up” camera. The lightreflecting back to the “look up” camera from all the surfaces frequentlyresults in some confusion regarding the exact location of the contactingportions 260. The inclined surface 304 of the devices 300 tends toreflect incident light away from lowerly disposed “look-up” cameras,while the contacting portions 306 tend to reflect incident light back tolowerly disposed “look-up” cameras. Light returning to the “look-up”camera primarily from the contacting portions 306 results in lesspotential confusion regarding the exact location of the contactingportions.

Ninth, the initial polishing of the top surface 202 of the substrate 200results in a matching smooth lower surface for the polyimide layer 220patterned thereon. After etching away, or otherwise removing, thesubstrate 200 the lower surface of the polyimide layer 220 is smooth andthe resulting polyimide layer 220 is generally optically clear.Accordingly, the spaces between the traces and the metallized devices300 is relatively optically transmissive so that an operator positioningthe device can readily see through the device between the traces anddevices. This assists the operator in manually positioning the membraneprobe on the devices which are otherwise obscured. In addition, thepyramidal shape of the devices 300 allows the operator to more easilydetermine the exact location of the contacting portions relative to thecontact pads on the test device, which were previously obscured by thewide beam structures (relative to the contacting portions).

Tenth, referring to FIG. 22, the contacting portions 260 of the deviceare preferably constructed with an exterior surface of rhodium 340,which typically can be effectively plated to only approximately athickness of 5 microns. The plating process of rhodium issemi-conformal, so the resulting layer is approximately 5 microns thickin a perpendicular direction to the exterior sides 352 and 354. Thewidth of the top 350 of the contacting portion and the angle of thesides 352 and 354 of the tool 210 is selected so that the rhodium 340plated on both sides 352 and 254 preferably join together forming av-shape. The remainder of the device is preferably nickel. While thethickness of the rhodium 340 is only 5 microns in a perpendiculardirection, the thickness of the rhodium 340 in a perpendicular directionfrom the top 350 of the device is greater than 5 microns. Accordingly,the contacting portion which wears during use in a generallyperpendicular direction from the top 350 will last longer than if thetop portion were merely plated to a thickness of 5 microns of rhodium.

Eleventh, the texture of the contacting portion 260 may be selected toprovide the described scrubbing effect on the contact pads of the testdevice. In particular, the tool may include a roughened surface patternon the corresponding contacting portion to provide a uniform texture forall devices.

Thirteenth, using the construction technique of the present invention isrelatively quick to construct the devices because of the decreasednumber of processing steps, resulting in a substantial cost savings.

The aforementioned construction technique also provides severaladvantages related to the shape of the devices which would be otherwisedifficult, if not impossible, to construct.

First, the tool may provide any desired shape, such as a simple bump, ifno scrubbing action is desired.

Second, the inclined supporting sides of the test device up to thecontacting portion 260 provides superior mechanical support for thecontacting portion 260, as opposed to merely a portion of metalsupported by a larger contact bump. With such support from the inclinedsides, the contacting portion may be smaller without risk of it becomingdetached from the device. The smaller contacting portion providesimproved contact with the contact pad of the test device when the devicetilts to penetrate the oxide buildup on the surface of the contact pad.In addition, the tail 302 of the device may be substantially thinnerthan the remainder of the device which decreases the likelihood of thetail 302 portion impacting the contact pad of the test device duringtesting when the device tilts.

Third, the pressure exerted by the contacting portions of the devices,given a predefined pressure exerted by the probe head, is variable bychanging the center of rotation of the device. The center of rotation ofthe device can be selected by selecting the length of the device and thelocation/height of the contacting portion relative thereto. Accordingly,the pressures can be selected, as desired, to match characteristics oftwo different contact pads.

Fourth, referring to FIG. 23, a triangular shape of the footprint of thedevice allows for high lateral stability of the devices while permittinga decrease in the pitch between devices. The contacting portions 403 ofthe device are preferably aligned in a linear arrangement for manycontact pads of test devices. The triangular portions of the device arealigned in alternatively opposing directions.

Fifth, the capability of constructing contacting portions that areraised high from the lower surface of the device, while stillmaintaining uniformity in the device height and structural strength,allows the device to provide scrubbing action while the lower surface ofthe device requires little movement. The small movement of the lowersurface of the device to make good electrical contact during testingdecreases the stress on the layers under the lower surface of thedevice. Accordingly, the likelihood of cracking the polyimide layers andthe conductive traces is reduced.

When probing an oxide layer on solder bumps, or solder balls on wafersthat are to be used with “flip-chip” packaging technology, such as thesolder bumps on the printed circuit boards, the oxide layer developedthereon is difficult to effectively penetrate. Referring to FIG. 24,when contacting a traditional contacting portion of a membrane probeonto the solder bump, the oxide 285 tends to be pressed-into the solderbump 287 together with the contacting portion 289 resulting in a poorinterconnection. When using conventional needle probes on solder bumps,the needles tend to skate on the solder bumps, bend under within thesolder bumps, collect debris on the needles, flake the debris onto thesurface of the test device, and cleaning the needle probes is timeconsuming and tedious. Moreover, needle probes leave non-uniform probemarks on the solder bumps. When probing solder bumps used on flip-chips,the probe marks left in the upper portion of the solder bump tends totrap flux therein, which when heated tends to explode, which degrades,or otherwise destroys, the interconnection. Referring to FIGS. 25 and26, an improved device construction suitable for probing solder bumps isshown. The upper portion of the device includes a pair of steeplyinclined sides 291 and 293, such as 15 degrees off vertical, withpreferably polished sides. The inclined sides 291 and 293 preferablyform a sharp ridge 295 at the top thereof. The angle of the sides 291and 293 is selected with regard to the coefficient of friction betweenthe sides and the oxide on the solder bump, so that the oxide coatedsurface tends to primarily slide along the surfaces of the sides 291 and293, or otherwise shear away, and not be significantly carried on thesides as the device penetrates a solder bump. Referring to FIG. 27, thesubstantially sharp ridge also provides for a mark (detent) aftercontact that extends across the entire solder bump. Subsequent heatingof the solder bumps, together with flux, result in the flux exiting fromthe sides of the solder bump thereby avoiding the possibility ofexplosion. In addition, the resulting mark left on the solder bumps isuniform in nature which allows manufacturers of the solder bumps toaccount for the resulting marks in their design. Also, less force isrequired to be applied to the device because it tends to slice throughthe solder bump rather than make pressing contact with the solder bump.The flatter surface 405 prevents slicing too deeply into the solder ball(bump).

Referring to FIG. 28, to provide a larger contact area for testingsolder bumps a waffle pattern may be used.

Referring to FIG. 29, an alternative device includes a pair ofprojections 311 and 313 that are preferably at the ends of an arch 315.The spacing between the projections 311 and 313 is preferably less thanthe diameter of the solder bump 317 to be tested. With such anarrangement the projections 311 and 313 will strike the sides of thesolder bump 317 thereby not leaving a mark on the upper portion of thesolder bump 317. With marks on the sides of the solder bump 317, thesubsequent flux used will be less likely to become trapped within themark and explode. In addition, if the alignment of the device is notcentered on the solder bump 317 then it is highly likely that one of theprojections 311 and 313 will still strike the solder bump 317.

Previous device construction techniques resulted in devices thatincluded contacting portions that were rather large and difficult toassure alignment of. Referring to FIG. 30, with the improvedconstruction technique the present inventors came to the realizationthat membrane probes may be used to make a “true” Kevlin connection to acontact pad on the test device. A pair of devices 351 and 353 arealigned with their contacting portions 355 and 357 adjacent one another.With this arrangement one of the devices may be the “force” while theother device is the “sense” part of the Kelvin testing arrangement. Bothcontacting portions 355 and 357 contact the same contact pad on the testdevice. A more detailed analysis of Kelvin connections is described inFink, D. G., ed., Electronics Engineers' Handbook, 1st ed., McGraw-HillBook Co., 1975, Sec. 17-61, pp. 17-25, 17-26, “The Kelvin DoubleBridge”, and U.S. patent application Ser. No. 08/864,287, both of whichare incorporated by reference herein.

It is to be noted that none to all of the aforementioned advantages maybe present in devices constructed accordingly to the present invention,depending on the technique used, desired use, and structure achieved.

1. A method of constructing a membrane probe comprising: (a) creating adepression in a substrate having a geometry substantially pyramidal inshape, wherein said substrate has a crystal grain and said depressionhas at least one substantially flat surface inclined relative to saidcrystal grain, said surface and said crystal grain defining an acuteangle therebetween; (b) locating conductive material within saiddepression, a conductive trace electrically connected to said conductivematerial, and membrane supporting said conductive material; (c) removingsaid substrate from said conductive material; and (d) roughening anouter surface of said conductive material.